[mlir][GPU] Expose the functionality to create a GPUFuncOp from a LaunchOp

The current setup of the GPU dialect is to model both the host and
device side codegen. For cases (like IREE) the host side modeling
might not directly fit its use case, but device-side codegen is still
valuable. First step in accessing just the device-side functionality
of the GPU dialect is to allow just creating a gpu.func operation from
a gpu.launch operation. In addition this change also "inlines"
operations into the gpu.func op at time of creation instead of this
being a later step.

Differential Revision: https://reviews.llvm.org/D75287